Particularly at airports, it is often necessary or desirable to screen passenger baggage for potential threats or contraband items. X-ray baggage inspection systems typically operate by exposing an item of baggage to X-ray radiation and detecting the X-ray radiation that is transmitted through or scattered from the examined baggage. Some systems have used a single view source detector arrangement, while others have used dual view or multi-view arrangements. The single or dual view systems usually scan baggage as it moves on a conveyor, using a fan or scanning pencil source beam of X-ray radiation in a fixed geometry. Multi-view systems such as Computed Tomography (CT) systems usually perform a 360° scan of stationary baggage, and process data corresponding to absorption of the X-ray radiation from different scan angles to reconstruct a three-dimensional image of the baggage.
Conventional X-ray detection systems usually include an X-ray source and an X-ray detector located at an inspection region, and a conveyor which moves an object to be inspected through the inspection region. The X-ray source exposes an object under inspection to X-ray radiation at one or more energies. The X-ray detector detects X-ray radiation either transmitted through or scattered by the object under inspection to provide X-ray data. The X-ray detector may include a crystal scintillator formed of an array of cells which detect X-ray radiation and convert it to light. The array may be one- or two-dimensional, usually depending on whether the X-ray source produces a pencil beam or fan beam of X-ray radiation. Conventional dual energy X-ray detection systems require two separate X-ray detectors, one sensitive to higher energy X-rays, and another sensitive to lower energy X-rays.
Conventional X-ray systems sample the X-ray detector cells every 2-4 milliseconds (ms), and thus integrate the signal received from about 20,000 individual X-ray pulses during the sample period of approximately 4 ms. Because electronic noise is also accumulated during the sample period, the signal-to-noise ratio (SNR) obtained for highly attenuative objects may be too low for the system to accurately scan the objects for certain applications, such as airport security.
Cell to cell variability in an X-ray detectors is also common. This variability may be caused by flaws in the detector crystals, and differences in the decay time, light collection efficiency or temperature for the individual cells. Where these variations exist, two X-ray photons may deposit equivalent amounts of energy within the detector crystal, but the overall electrical response of the detector cell may be different. It may be desirable to be able to compensate for such variability so as to obtain better image resolution.